Long-term corrosion resistance of Al–Ni-plated material and Al-plated material in molten carbonate environment

Al–Ni-plated material, which was produced by Ni and Al electroplating on a Type 310S stainless steel substrate followed by heat-treating at 1023 K, has been developed for the wet-seal material of MCFC. In order to compare the corrosion resistance of Al–Ni-plated material with that of Al-plated material, which simulated the Al-sprayed material presently used as wet-seal material, an immersion test was carried out in molten carbonate in 70% air–30% CO2 gas at 923 K for 14,500 h. Both Al–Ni-plated material and Al-plated material showed good corrosion resistance during this test period. From the analyses of the surface, in cross-section, of these materials after the test, however, it was confirmed that a large amount of Al in Al–Ni-plated material was retained as an Al–Ni intermetallic compound under the LiAlO2 layer, while the Al in Al-plated material tended to diffuse from the surface into the substrate during the test. It is suggested that the lifetime of the Al–Ni-plated material is longer than that of the Al-plated material, since Al–Ni intermetallic compounds in the Al–Ni-plated material provide a supply of Al to the LiAlO2 layer, the stability of which leads to prolonged corrosion resistance of the wet-seal material to molten the carbonate. Therefore, it is expected that the Al–Ni-plated material maintains a higher corrosion resistance, compared with the present material used for the wet-seal area of MCFC.